The cellular mobile telephone system uses two different modes of FM operation: a wide band FM modulation mode; and a narrow band FM modulation mode. Wide band modulation provides greater noise immunity but narrow band modulation provides a greater number of channels per a given band of frequencies. In order to be usable regardless of the operation mode a cellular mobile telephone needs to be able to accommodate both modes of operation. The received FM signal, whether wide band or narrow band, and whether being acted upon by a cellular mobile telephone or by a base station of a mobile switching center (MSC), is typically first converted to an intermediate frequency (IF) signal and then demodulated to recover the audio signal represented by the received FM signal. An FM quadrature detector is commonly used to demodulate the FM IF signal and provide a demodulated output signal. However, a quadrature detector configured to optimally detect wide band signals will provide a very low demodulated output signal when the received signal is a narrow band signal. Furthermore, a quadrature detector configured to optimally detect narrow band signals will provide a distorted or unusable demodulated output signal when the received signal is a wide band signal. Therefore, the detector is typically configured for wideband operation and, for narrowband operation, the gain of a subsequent amplifier stage is increased or an additional amplifier is switched into the signal path. However, this has the disadvantage in that the desired signal is narrow band, but noise is wideband and may be present at frequencies which give a greater output voltage than the desired signal, so noise immunity is compromised. Also, two separate detectors may be used, one for demodulating wide band signals and one for demodulating narrow band signals. However, the above methods suffer from one or more of the following disadvantages: duplication of parts; extra space for the circuitry; increased costs; and increased power consumption. Therefore, there is a need for an FM quadrature detector which will optimally respond to both narrow band and wide band FM signals.